<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
body {
  background-color: red;
}

@media only screen and (max-width: 900px) {
  body {
    background-color: lightblue;
  }
}
</style>
</head>
<body>

<p>请调整浏览器窗口的大小。如果此文档的宽度为 600 像素或更小，背景颜色为“浅蓝色”，否则为“浅绿色”。</p>

</body>
</html>
